As the demand for complex machined parts has dramatically increased in recent years, technology has evolved to create such intricate and precise parts. One technology that is now widespread for creating such intricate and precise parts is the technology known as “wire electrostatic discharge machining (wire EDM).” This technology permits the machining of complicated geometric shapes from hard and difficult to cut electrically conductive materials.
Wire EDM is used for cutting and shaping many types of material such as sheet metal parts. The wire EDM uses a small diameter wire that moves like the blade in a band saw, except that the wire is discarded after one pass through the work. The wire is supported above and below the work piece and is fed from above the work piece to below as machining of the work piece proceeds. As the sheet of metal moves automatically in response to a NC (numerical control) program, the electrical discharge moves atoms of the metal from the work piece to the wire in a kind of reverse welding process. An EDM rough cut may take between eight and 12 hours with the wire moving only a few thousandths of a millimeter at a time. The finer the pass of the wire, the faster the wire moves; and thus the wire is continuously moving past the work piece. This degrades the wire the longer a portion of the wire is maintained in spark gap contact with the work piece. For thick work pieces the accuracy of the rough pass will degrade along with the wire as the wire moves past the work piece thereby requiring more skim passes. In addition to shaping sheet metal, wire EDM is also used in the finishing processes for various types of plastic forming molds, press casting molds, forging molds, etc.
One of the most critical factors in the wire EDM process is properly locating and positioning the work piece on or within the fixture, plate, or work platform at what is referred to as “the working origin” so that alignment precision can be maintained throughout the wire EDM process. Moreover, it is often necessary to remove the work piece for temporary checking and inspection; and then the work piece must be returned to the fixture with no loss of alignment precision either by the work piece or the wire. Most tooling currently available utilizes a type of 3 axes positioning and adjustment (the x, y, and z axes). This is a useful feature when used in mass production runs; however, when applied to the production of precision parts, the problem of accurate repeatability in alignment and positioning of the work piece and the discharge wire arises. Thus, the prior art reveals a range of devices, assemblies, and techniques for locating, positioning and aligning the work piece and the discharge wire. The problem of obtaining exact repeatability in locating the work piece is often is due to the fact that leveling (of the work piece on the fixture and the fixture with respect to the machine rail) is accomplished by the use of jackscrews or cams. The fact that the fixture holding the work piece is not directly attached to the machine rail creates certain variables that affect repeatability in the positioning of the work piece.
For example, the Johanson Patent (U.S. Pat. No. 5,019,219) discloses a work piece holding system that includes opposed vises that exert a downward force to the work piece, and the vises include dovetail recesses for holding tall or elongate work pieces.
The Buhler et al. Patent (U.S. Pat. No. 5,438,178) discloses a method of precision electric-discharge machining wherein a wire electrode is used for producing at least one finishing stroke on a work that has already been roughed out at least once.
The Tibbet Patent (U.S. Pat. No. 5,487,538) discloses a work piece holding assembly for holding a work piece that includes a rail clamp, a beveled head and a beveled rail that adjustably interconnect for mounting to a machine table in order to hold the work piece.
The Iwasaki Patent (U.S. Pat. No. 5,095,635) discloses a positioning block for the wire of an NC wire electro-spark processing machine that includes a block that is used to determine the position of the discharge wire in the coordinate system in which the table and the work piece move.
The Tibbet Patent (U.S. Pat. No. 5,595,377) discloses a work piece presetting assembly that includes a subassembly for precisely aligning a work piece to be worked on along predetermined orthogonal axes.
The Lai Patent (U.S. Pat. No. 6,091,041) discloses an electric discharge machine that includes a movable work platform for supporting a cylindrical work piece so that the work piece can be flattened for finishing by a discharge finishing electrode.
The Nordquist Patent (U.S. Pat. No. 6,103,987) discloses a retaining plate for a machine tool work piece that includes at least one opening for receiving the work piece and sections that allow for the transference of the retaining plate without losing alignment precision.
The Asai et al. Patent (U.S. Pat. No. 6,179,279 B1) discloses a gripping device for holding a work piece that includes a movable block disposed between fixed blocks with the movable block and at least one fixed block capable of holding irregular-shaped work pieces.
Nonetheless, despite the ingenuity of the above devices there remains a need for an assembly for use with a wire EDM machine that can maintain extremely close tolerances and that permits removal of the work piece from the fixture for checking and evaluation, so that the return of the work piece to the fixture can be done with maximum repeatability to the exact location on the fixture.